Landslides of the Middle Reventazón Catchment—Lithological and Topographic Controls

Many methodologies developed to establish susceptibility or probability to slope instability prioritize the slope as the main passive factor that triggers movement. However, landslides along the Reventazón RiverReventazón river, between Angostura of Turrialba and Siquirres, have been observed on low-angle slopes. These slopes show movements even in areas of less than 20°. They were influenced by the passive factor of lithology. Based on a bibliographic review and field work, the Uscari Formation, composed of sandstones and fissile shales, has been identified as a relevant factor in the generation of many of the landslides that occur on the slopes of the Reventazón RiverReventazón river. This lithology, presenting clays and high friability, moves downslope in the presence of water. On the other hand, there are volcanic lithologies and denser colluviums on top of the Uscari Formation, exerting weight on the unstable mass and, therefore, allowing for mass displacements. The areas of the San Antonio, Jesús María, Torito, or Tres Equis landslides represent intensive mass movement processes on gentle slopes, the result of which is very important in the modeling of the relief. The associated destruction has been extreme for the case of the Jesús María landslide, which destroyed in a timespan of two decades, the homonymous town.